Cambodia's united Nations-backed genocide tribunal yesterday formally charged the four top surviving leaders of the Khmer Rouge regime blamed for 1.7 million deaths in the 1970s, paving the way for the panel's long-awaited second trial next year.

The frail, elderly defendants, who have been in detention since 2007, deny any guilt for their roles in the radical communist rule during which about a quarter of Cambodia's population was either executed or died due to starvation or overwork.

The trial, due to start by mid-2011, will bring to the stand Nuon Chea, 84, the group's ideologist; Khieu Samphan, 79, the former head of state; Ieng Sary, former Foreign Minister, and his wife Ieng Thirith, ex-minister for social affairs, both in their 80s. Each faces four charges: crimes against humanity, genocide, war crimes and a combined charge of murder, torture and religious persecution, said co-investigating Judge You Bunleng. They will be tried together.

Several other major Khmer Rouge figures have died, including supreme leader Pol Pot in 1998, creating pressure to expedite proceedings. The charges follow July's conviction of the regime's chief jailer, Kaing Guek Eav, known as Duch, on charges of war crimes, crimes against humanity, torture and murder. He was sentenced to 19 years in prison.